Particle characterization apparatus and method

Abstract

An apparatus is provided for determining particle characteristics, in which a flow path is generated containing particles to be analyzed. A light detection system detecting light received from a measurement zone which has been scattered by the particles. A time duration for which a particle remains in the measurement zone is measured to determine an effective aerodynamic particle diameter and a peak detected received light intensity is measured to determine an effective optical particle diameter. A further particle parameter is also obtained relating to the shape and/or density of the particle. This approach enables more information than only a particle size to be obtained using a single-stage optical analysis system. The additional information may be used to characterize the particles more accurately.

Claims

1. An apparatus for determining particle characteristics, the apparatus comprising: an inlet and an outlet; a flow system for generating a flow path between the inlet and the outlet along which an accelerating flow is to be provided containing particles to be analyzed; a light source for providing light to the flow path, wherein the envelope of the light defines a measurement zone, the measurement zone having a length of more than 1 cm; a light detector for detecting light received from the measurement zone which has been scattered by the particles; and a controller for analyzing the detected received light, wherein, for a particle of the particles, the controller is adapted to: determine an effective aerodynamic particle diameter of the particle from a time duration for which the particle remains in the measurement zone; determine an effective optical particle diameter of the particle from a peak detected received light intensity; analyze a determined effective aerodynamic particle diameter and a determined effective optical particle diameter; and determine a particle parameter relating to at least one of a shape of the particle and a density of the particle from an analysis of the determined effective aerodynamic particle diameter and the determined effective optical particle diameter.

2. The apparatus as claimed in claim 1, wherein the controller is further adapted to: determine the particle parameter as a ratio between a shape parameter of the particle and the density of the particle derived from the analysis of the determined effective aerodynamic particle diameter and the determined effective optical particle diameter.

3. The apparatus as claimed in claim 2, wherein the controller is adapted to: derive a shape parameter of the particle from a level of variation of the detected received light intensity over time; and derive the particle density of the particle from a derived shape parameter of the particle.

4. The apparatus as claimed in claim 1, further comprising: a first polarizer between the light source and the measurement zone; and a second polarizer between the measurement zone and the light detector, wherein the second polarizer has a first portion with a matching polarization to the first polarizer and a second portion with an orthogonal polarization to the first polarizer.

5. The apparatus as claimed in claim 4, wherein the controller is adapted to: derive a shape parameter of the particle from the peak detected received light intensity through the first and second portions of the second polarizer; and derive the particle density of the particle from a derived shape parameter of the particle.

6. The apparatus as claimed in claim 1, wherein the flow system includes a fan connected to the outlet, and wherein the apparatus further comprises: an outer enclosure; a filter arrangement coupled to a further pair of inlets; and a flow deflector arrangement for controlling the flow from the filter arrangement and for controlling the flow path.

7. The apparatus as claimed in claim 6, wherein the filter arrangement includes first and second filters on opposite sides of the flow path; and wherein the flow deflector arrangement includes corresponding first and second flow deflectors.

8. The apparatus as claimed in claim 1, wherein the flow system is adapted to provide a uniform acceleration of the flow along the flow path within the measurement zone.

9. A method for obtaining particle characteristics, the method comprising: generating an accelerating flow containing particles along a flow path between an inlet and an outlet; controlling a light source to provide light to the flow path, wherein the envelope of the light defines a measurement zone, the measurement zone having a length of between 1 cm and 6 cm; detecting light received from the measurement zone which has been scattered by the particles; determining an effective aerodynamic particle diameter of a particle of the particles from a time duration for which the particle remains in the measurement zone thereby; determining an effective optical particle diameter of the particle from a peak detected received light intensity; analyzing a determined effective aerodynamic particle diameter and a determined effective optical particle diameter; and determine a particle parameter relating to at least one of a shape parameter of the particle and a particle density of the particle from the analyzing of the determined effective aerodynamic particle diameter and the determined effective optical particle diameter.

10. The method as claimed in claim 9, wherein the particle parameter is determined as a ratio between a shape parameter of the particle and the density of the particle from the analyzing of the determined effective aerodynamic particle diameter and the determined effective optical particle diameter.

11. The method as claimed in claim 10, further comprising: analyzing a level of variation of the detected received intensity over time to derive a shape parameter of the particle; and deriving the density of the particle from the derived shape parameter of the particle.

12. The method as claimed in claim 9, further comprising: providing polarization of the light between before the measurement zone using a first polarizer; providing polarization of the light after the measurement zone using a second polarizer having a first portion with a matching polarization to the first polarizer and a second portion with an orthogonal polarization to the first polarizer; analyzing the peak detected received light intensity through the first and second portions of the second polarizer to derive a shape parameter of the particle; and deriving the density of the particle from the derived shape parameter of the particle.

13. The method as claimed in claim 9, further comprising: filtering an air flow towards the measurement zone to thereby to control the flow path.

14. The method as claimed in claim 9, further comprising: providing a uniform acceleration of the flow along the flow path within the measurement zone.

15. A computer program comprising computer program code means which is adapted, when said computer program is run on a computer, to perform the method of claim 9.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Examples of the invention will now be described in detail with reference to the accompanying drawings, in which:

(2) FIG. 1 shows a first possible way to obtain information relating to particles in addition to the particle diameter;

(3) FIG. 2 shows a second possible way to obtain information relating to particles in addition to the particle diameter;

(4) FIG. 3 shows a first example of a particle size determining apparatus and shows the parts relating to the optical system;

(5) FIG. 4 shows an example of the collected light intensity signal;

(6) FIG. 5 shows the parts of the apparatus relating to flow control;

(7) FIG. 6 shows the flow paths within the apparatus;

(8) FIG. 7 shows the velocity profile within the measurement zone;

(9) FIG. 8 shows the effect of particle density and hence the particle aerodynamic diameter on the velocity reached in the accelerating flow field;

(10) FIG. 9 shows the terminal velocity versus the particle diameter.

(11) FIG. 10 shows the sensitivity of the terminal velocity versus the particle diameter.

(12) FIG. 11 shows the pulse width versus the particle diameter.

(13) FIG. 12 shows the sensitivity of the pulse width versus the particle diameter;

(14) FIG. 13 shows a collected signal for a non-spherical particle;

(15) FIG. 14 shows a collected signal for a spherical particle;

(16) FIG. 15 shows an apparatus which uses polarization to enable determination of a shape parameter;

(17) FIG. 16 shows an example of the collected light intensity for a non-spherical particle using the apparatus of FIG. 15;

(18) FIG. 17 shows parameters associated with the optical arrangement of FIG. 3 and shows how decreasing the angle of incidence .sub.mc can increase irradiance while maintaining the length of the measurement zone L; and

(19) FIG. 18 shows a method of determining particle characteristics.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(20) The invention provides an apparatus for determining particle characteristics, in which a flow path is generated containing particles to be analyzed, in which an accelerating flow is provided. A light detection system detects light received from a measurement zone which has been scattered by the particles. A time duration for which a particle remains in the measurement zone is measured to determine an effective aerodynamic particle diameter and a peak detected received light intensity is measured to determine an effective optical particle diameter. A further particle parameter is also obtained relating to the shape and/or density of the particle.

(21) This approach enables more information than only a particle size to be obtained using a single-stage optical analysis system. The additional information may be used to characterize the particles more accurately.

(22) FIGS. 1 and 2 show examples of possible configurations for obtaining information additional to an equivalent diameter measurement.

(23) FIG. 1 shows how multiple measurements may be used in sequence to obtain a particle density measurement.

(24) The system comprises a differential mobility analyzer 10 to which a sample flow 12 is provided. This provides an electrical mobility diameter EMD as its output. The flow passes to an optical particle counter 14 which provides an optical diameter OD as its output. The two outputs are processed by processor 16 which provides a density measurement D.

(25) FIG. 2 shows how multiple measurements may be used in sequence to obtain a shape factor (which is an indication of how far the shape deviates from a perfect sphere).

(26) The system comprises an aerosol particle mass analyzer 20 to which a sample flow 22 is provided. This provides an aerosol mass AM as its output. The flow passes to a scanning mobility particle sizer 24 which provides an electrical mobility diameter EMD as its output. The two outputs are processed by processor 26 which provides a shape factor measurement SF.

(27) A scanning mobility particle sizer essentially performs the same function as a differential mobility analyzer but it can also select particles with multiple electrical mobility values by changing the electric field intensity.

(28) These approaches thus require complicated apparatus in order to enhance the basic measurement of effective diameter.

(29) The invention is instead based on the use of a single optical stage with a wide light source beam such as a laser beam to measure particle effective optical diameter (based on the intensity of a pulse received by the photon detector) and particle transit time and/or terminal velocity (based on the width of the pulse) in a steady state accelerating flow field. By steady state is meant that the flow conditions are temporally constant, i.e. the flow has the constant velocity (and acceleration) at different points along the flow path over time.

(30) The wide light source beam defines a measurement zone. In particular, the envelope the light source beam determines the length of the measurement zone. This means that the transit time through the measurement zone can be determined based on analysis of reflected light, and does not require any additional timing measurements.

(31) Thus, separate components are not needed for a time of flight measurement. Only an incident beam, for example generated by a laser diode and beam shaping optics, is needed.

(32) A pre-defined relationship may be used to convert the transit time and/or terminal velocity to an effective aerodynamic diameter. The transit time is more easy to measure as it related directly to the signal width. The effective aerodynamic and optical diameters may then be processed to yield additional information about the particle, such as one or more of the particle density, shape factor, type and composition.

(33) By way of example, a laser beam with width 4 cm is wide enough to identify differences in particle terminal velocity and flight time induced by differences in particle effective aerodynamic diameter, and hence particle density and composition. More generally, the measurement zone has a length more than 1 cm, for example a spatial length in the range 1 cm to 6 cm. The larger the length, the greater the sensitivity, so there is a trade-off between sensitivity and sensor size.

(34) This additional information can thus be obtained without requiring multiple sensing instruments and enables portable sensors to obtain particle size, type and composition with reasonable accuracy in a quick and cost-effective manner.

(35) The invention combines optical sensing with flow control.

(36) FIG. 3 shows the optical sensing features.

(37) The optical apparatus comprises an inlet 30 which connects to the particle source. A flow path is defined to an outlet 32 which is connected to a negative pressure source 33 such as a fan, to control the flow.

(38) A laser diode 34 provides an illumination beam, which is collimated by collimator lens 36 and then illuminates a length of the flow path. The envelope of the laser signal defines the length of the flow path which in turn defines a measurement zone. A photon detector 38 such as an avalanche photodiode collects scattered light after focusing by lens 40.

(39) The light for example has a top-hat intensity profile. A lenslet array or a Powell lens may be used to convert a Gaussian laser output into incident light with a uniform intensity.

(40) The photon detector signal is provided to a controller 41, which also controls the laser source 34 and the fan 33. The controller outputs the effective optical diameter d.sub.op the effective aerodynamic diameter d.sub.ac and one or more further parameters such as a shape factor and a density .sub.p or a ratio of such values. It may also output an identification of the particle type by mapping the particle characteristics to known pollutants using a database.

(41) FIG. 4 shows an example of a signal recorded by the photon detector 38 over time. The pulse relates to the transit of a single particle. It has a maximum intensity I.sub.max and a time duration t between times t.sub.1 and t.sub.2. The measurement zone corresponds to the position of the particle between times t.sub.1 and t.sub.2.

(42) The light is collected at a pre-defined angle. The light intensity signal of FIG. 4 is converted to digital form using an analog to digital converter and recorded as a time series, within the controller 41.

(43) Due to the constraints of laser power and analog to digital conversion resolution and signal to noise ratio, a low angle between the incident laser light and particle beam is used so that the laser power is concentrated into a small area but a sufficient length of illumination of the measurement zone is enabled. This is explained further below. The use of an avalanche photon detector enables an increase in the sensitivity of the scattered light intensity measurement.

(44) The width of the pulse t in FIG. 4, i.e. the transit time, relates to the velocity of the particle in the laser beam, which is determined by the aerodynamic diameter of the particle, while the height of the signal I.sub.max represents the light intensity that is scattered off the particle, which is proportional to the effective optical diameter of the particle.

(45) Thus, the single optical measurement provides multiple sources of information concerning the particle characteristics. In particular, by analyzing the two effective diameters, one or more of the particle density, shape factor and particle type can be determined.

(46) FIG. 5 shows the flow control apparatus.

(47) It shows the inlet 30 and outlet 32 of FIG. 3. The inlet is at atmospheric pressure, and the outlet is connected to the fan, such as a centrifugal fan (with a higher static pressure at low flow rate compared to a coaxial fan). The inlet and outlet pass into an enclosure 42, and filters 44 are provided within the enclosure, in particular on opposite sides of the flow channel.

(48) The filters 44 are open to the exterior, so that they present a flow path from the outside to the inside of the enclosure 42. Thus, the flow out of the enclosure 42 is balanced by the flow in through the two filters and the flow through the inlet 30. The flow rate through the inlet 30, and thus the dilution rate of the incoming particle-laden air flow, depends on the characteristics of the filters, e.g. size, thickness, resistance coefficient etc.

(49) Flow deflectors 46 are provided for shaping the flow streamlines to focus the particle trajectory from the inlet.

(50) The design of the aerodynamic flow control apparatus part aims to generate a steady state accelerating flow field inside the measurement zone.

(51) Particles with different aerodynamic diameter accelerate to different extent in an accelerating flow. This property is thus used to perform particle sizing with respect to aerodynamic diameter. Using a uniform flow field cannot achieve this goal, as all particles will settle to a same velocity profile in the measurement zone, and thus the same transit time.

(52) The filters create a clean environment in the measurement zone to eliminate contamination by unwanted particles during light scattering measurements. This is especially helpful at each start-up of the system after it has been idle for a long time. They also dilute the incoming particle-laden air flow to ensure that, during any time, only one particle is present in the measurement zone (i.e. within the laser beam path). The dilution rate can be adjusted by changing the flow resistance (i.e. the material or thickness) of the filtration material.

(53) The negative pressure induced by the fan 33 is set for example down to approximately 50 Pa.

(54) FIG. 6 shows how the flow is controlled.

(55) The flow has a reducing cross sectional area from the inlet 30 to the outlet 32, hence an increasing velocity along its axis. The air flow 50 through the filters 44 focuses the particle trajectory, and there is an acceleration towards the outlet 32 within the measurement zone 52 where the measurement takes place.

(56) FIG. 7 shows the flow velocity profile in the measurement zone 52, as a plot of velocity versus distance along the flow path.

(57) FIG. 8 shows the effect of particle density (which correlates to the effective aerodynamic diameter) on the velocity reached within the acceleration zone 52. It shows the relative velocity versus the distance from the inlet nozzle for a set of different particle densities ranging from 1.0 g/cm.sup.3 to 2.0 g/cm.sup.3 for particles with identical diameter of 10 m, and uses particles of density 1.0 g/cm.sup.3 as a baseline reference.

(58) From this graph, the velocity difference with respect to the baseline can be determined (such as the vertical arrow 80 shown) at different distances from the inlet nozzle. The reference line (v=0) indicates the reference with particles having density 1 g/cm.sup.3, and thus is set to 0 at all distances.

(59) The other plots show the velocity difference for particles with density 1.2, 1.5, 1.8 and 2.0 g/cm.sup.3 compared to the baseline reference. Larger particles accelerate more slowly than smaller particles, thus particles with higher density (i.e. larger aerodynamic diameter) have a more negative value as shown by arrow 80 for the most dense particle.

(60) After the accelerating flow field, in the measurement zone, the velocity profile of the flow changes, so that the velocity differences then diminish and all particles settle in the flow field with the same terminal velocity. This is also shown in FIG. 8.

(61) As explained above, the total time a particle traverses the measurement zone (i.e. the width of the signal t in FIG. 4) is used to characterize the aerodynamic diameter of the particle. To estimate the potential accuracy and stability of this algorithm, a sensitivity analysis is performed for the two relationships (i) terminal velocity to particle effective aerodynamic diameter, and (ii) transit time (in the measurement zone) to particle effective aerodynamic diameter.

(62) The sensitivity (x) can be defined as [(dy/y)/(dx/x)] (x) for a relationship y=f(x).

(63) This sensitivity defines the ratio of percentage of change in y and percentage of change in x at different values of x. A high sensitivity means that a change in x will cause a prominent change in y. Thus by measuring y, a more accurate determination of the value of x can be obtained with less interference from noise.

(64) The sensitivity analysis is performed for particles with aerodynamic diameter ranging from 1 m to 100 m in steps of 1 m and a uniform density 1.510.sup.3 kg/m.sup.3. The measurement zone length is 4 cm.

(65) FIG. 9 shows the terminal velocity v (m/s) versus the particle diameter (m).

(66) FIG. 10 shows the sensitivity S (unitless) of the terminal velocity versus the particle diameter (m).

(67) FIG. 11 shows the pulse width W (ms) versus the particle diameter (m).

(68) FIG. 12 shows the sensitivity S (unitless) of the pulse width versus the particle diameter (m).

(69) In FIG. 10, the sensitivity of the relationship particle between the terminal velocity and particle effective aerodynamic diameter ranges from 0 to 0.5 (absolute value) with higher sensitivity for larger particles. The same conclusion can be drawn from FIG. 12 where the sensitivity of the relationship between particle transit time and particle effective aerodynamic diameter ranges from 0 to 0.25, also with larger particles having higher sensitivity.

(70) This sensitivity analysis shows that the use of particle transit time to characterize particle aerodynamic is feasible and more sensitive (giving a more accurate and stable estimation) for larger particles.

(71) The nature of the movement of the particles in the flow can be analyzed based on the drag force experienced by the particles.

(72) The drag force is given by:

(73) $\begin{array}{cc}{F}_{\mathrm{drag}}=\frac{3V_{\mathrm{set}}{d}_p}{C_c\left(d_p\right)}& \left(1\right)\end{array}$

(74) For a particle of interest:

(75) $\begin{array}{cc}{}_p\frac{}{6}{d}_{\mathrm{op}}^{3}g=\frac{3V_{\mathrm{Set}}{d}_{\mathrm{op}}\mathrm{��}}{C_c\left(d_{\mathrm{op}}\right)}& \left(2\right)\end{array}$

(76) For a particle with standard density:

(77) $\begin{array}{cc}{}_0\frac{}{6}{d}_{\mathrm{ae}}^{3}g=\frac{3V_{\mathrm{Set}}{d}_{\mathrm{ae}}}{C_c\left(d_{\mathrm{ae}}\right)}& \left(3\right)\end{array}$

(78) These two relationships can be combined to yield:

(79) $\begin{array}{cc}{d}_{\mathrm{ae}}=d_{\mathrm{op}}\sqrt{\frac{1}{\mathrm{��}}\frac{{}_p}{{}_0}\frac{C_c\left(d_{\mathrm{op}}\right)}{C_c\left(d_{\mathrm{ae}}\right)}}& \left(4\right)\end{array}$
.sub.p is the particle density
.sub.0 is the standard density
is the dynamic viscosity of air=1.89310.sup.5 Pa.Math.s
is the dynamic shape factor (1 for a sphere, <1 for streamlined shape, >1 for most aerosol particles)
V.sub.set is the settling velocity (the relative velocity of the particle with respect to the carrier flow)
C.sub.c(d.sub.p) is slip correction factor, which has a known relationship with respect to d.sub.p and can be approximated to 1 for particles larger than 1 micrometer
d.sub.op is the effective optical diameter
d.sub.ae is the effective aerodynamic diameter
d.sub.p is the generic particle diameter value used in the generic drag equation.

(80) There are different options for processing the recorded data.

(81) In a most basic version, the height and width of the scattered signal of FIG. 4 are measured, where the height of the signal is related to particle effective optical diameter, and the width of the signal (the transit time of the particle in the laser beam) is related to the effective particle aerodynamic diameter by the relationship shown in FIG. 10.

(82) After obtaining the effective optical and aerodynamic diameters, the value under the root sign of Equation (4) can be derived for a specific particle.

(83) In addition, as Cc(d.sub.ae) and Cc(d.sub.op) have known relationship with dependent variable d (i.e. d.sub.ae and d.sub.op) and are close to one for micron-sized particles, the ratio of density (.sub.p) and shape factor () can be calculated as a single number.

(84) This most basic approach provides one additional parameter which combines density and shape factor. This may be used to provide particle differentiation in that different particles will have different values for this parameter.

(85) In this basic version, the measured variables are the effective optical diameter (derived from peak intensity) and the effective aerodynamic diameter (derived from the transit time). These are outputs from the system.

(86) Known variables are .sub.0=1 g/cm.sup.3, Cc(d.sub.op)1, Cc(d.sub.ae)1.

(87) The calculated additional variable is the ratio of particle density (.sub.p) to shape factor (). This is also provided as an output. The system can also output particle number concentration.

(88) It would be more useful to be able to separate the particle density and shape information. Thus, in a more advance implementation, a particle shape factor may be obtained by analyzing a variation of the scattered light signal collected by the photon detector.

(89) FIGS. 13 and 14 are used to explain a first approach.

(90) FIG. 13 shows a collected signal for a non-spherical particle. The orientation of the particle influences the scattered light intensity in the direction of the photon detector, and this manifests itself as a variation in the intensity level during the period of the signal corresponding to the central area of the measurement zone.

(91) FIG. 14 shows a collected signal for a spherical particle. The orientation of the particle does not influence the scattered light intensity in the direction of the photon detector, so the peak intensity has a period of constant amplitude. In each case, the peak intensity 120 recorded for the purposes of determining the effective optical diameter is the same.

(92) A shape factor x can then be evaluated and hence the particle density can be separated from the shape factor. The deviation of the signal of FIG. 13 from a constant peak value may be obtained by any suitable statistical analysis, such as the peak deviation from the baseline level (shown dotted) or an area of deviation.

(93) FIG. 15 is used to explain a second approach.

(94) The optical system is enhanced by providing a first polarizer 150 in the path of the laser beam, before the measurement zone, and a second polarizer 152 is provided in the path to the photon detector 38.

(95) The second polarizer has two portions 152a, 152b with orthogonal polarization. The first part 154 of the path of the particle within the measurement zone is detected based on light which has passed through the first portion 152a, and the second part 156 of the path of the particle is detected based on light which has passed through the second portion 152b. In this example, the first portion has a polarization aligned with the first polarizer, and the second portion has a polarization orthogonal to the first polarizer.

(96) FIG. 16 shows an example of the collected light intensity for a non-spherical particle.

(97) During a first part of the time period a light intensity is measured for light that has passed though aligned polarizers, with a maximum intensity shown as I.sub.max. During a second part of the time period a light intensity is measured for light that has passed though crossed polarizers, with a maximum intensity shown as I.sub.max.sub.. In this case, only light that has undergone a polarization rotation between the polarizers can be collected.

(98) For spherical particles, the scattered light will maintain its original polarization along each of the two orthogonal directions. For non-spherical particles, a portion of the scattered light will experience a change in polarization, hence yields a cross contribution of polarized components between the two orthogonal directions.

(99) The two polarized components of the scattering field depend on the S-matrix

(100) $\left(\begin{array}{c}{E}_{.Math.s}\\ E_s\end{array}\right)=\frac{e^{\mathrm{ik}\left(r-z\right)}}{-\mathrm{ikr}}\left[\begin{array}{cc}{S}_2& S_3\\ S_4& S_1\end{array}\right].Math.\left(\begin{array}{c}{E}_{.Math.i}\\ E_i\end{array}\right)$

(101) For spherical particles, S.sub.3=S.sub.4=0, so that the two polarization components in the incident and scattered light will not interact. For non-spherical particles, S3=S40, the two polarization components will interact. i.e. the parallel component in the incident light will contribute to both parallel and perpendicular components in scattered light.

(102) A shape measure may be defined as:

(103) $P\frac{I{\max }_{.Math.}-I{\max }_{}}{I{\max }_{.Math.}+I{\max }_{}}=f\left(\mathrm{��}\right)$

(104) For spherical particles, I.sub.max=0.Math.P=1. For non-spherical particles, Imax.sub.>0 .Math.P<1.

(105) Thus, the parameter P provides a measure of the shape of a particle from which an estimate is obtained for shape factor and hence the particle density can be separated from the shape factor.

(106) In this approach, the measured variables are the effective optical diameter (based on the peak intensity), the effective aerodynamic diameter (based on the transit time) and the shape factor () (based on the shape of the intensity plot). These are outputs by the system.

(107) The known variables are again .sub.0=1 g/cm.sup.3, Cc(d.sub.op)1, Cc(d.sub.ae)1.

(108) The calculated variable is the particle density (.sub.p) which is also output from the system, again also with a particle count.

(109) The retrieval of the effective aerodynamic diameter as well as the shape factor using either of these approaches requires a wide incident laser beam (of the order of millimeters or centimeters) compared to a typical narrow laser beam (of the order of micrometers).

(110) The left part of FIG. 17 shows parameters associated with the optical arrangement of FIG. 3, in particular the laser beam width d and the path length L in the measurement zone. The angle between the incident beam and the flow path is shown as .sub.inc. The scattering angle between the incident beam and the photon detector is shown as .sub.sca.

(111) The right part of FIG. 17 shows the effect of decreasing the angle of incidence.

(112) For a given path length L and laser power, and beam width d can be decreased, so that the irradiance can be increased giving a higher signal to noise ratio.

(113) The scattering angle .sub.sca can also be decreased to increase the scattering intensity reaching the photon detector, as there is more forward scattering for particles with size larger than 0.5 m.

(114) By way of example, preferred ranges for .sub.inc are 15 to 25 such as 20. Preferred ranges for .sub.sca are 20 to 40 such as 30. The length L has the order of centimeters, such as 1 cm to 6 cm, such as 2 cm. A suitable range for d is L/3 to L/4.

(115) The system will be calibrated with respect to:

(116) The signal height to effective optical diameter relationship;

(117) The signal width to effective aerodynamic diameter relationship;

(118) The signal variation to shape factor relationship (when used); and

(119) The parameter P to shape factor relationship (when used).

(120) FIG. 18 shows a method for obtaining particle characteristics, comprising:

(121) in step 180, generating an accelerating flow containing particles between an inlet and an outlet;

(122) in step 182, controlling a laser light source to provide light to a measurement zone of the flow path;

(123) in step 184, detecting light received from the measurement zone which has been scattered by the particles;

(124) in step 186, determining a time duration for which a particle remains in the measurement zone and thereby determining an effective aerodynamic particle diameter;

(125) in step 188, determining a peak detected received light intensity and thereby determining an effective optical particle diameter; and

(126) in step 190, determining a further particle parameter relating to the shape and/or density of the particle.

(127) As discussed above, embodiments make use of a controller 41. The controller can be implemented in numerous ways, with software and/or hardware, to perform the various functions required. A processor is one example of a controller which employs one or more microprocessors that may be programmed using software (e.g., microcode) to perform the required functions. A controller may however be implemented with or without employing a processor, and also may be implemented as a combination of dedicated hardware to perform some functions and a processor (e.g., one or more programmed microprocessors and associated circuitry) to perform other functions.

(128) Examples of controller components that may be employed in various embodiments of the present disclosure include, but are not limited to, conventional microprocessors, application specific integrated circuits (ASICs), and field-programmable gate arrays (FPGAs).

(129) In various implementations, a processor or controller may be associated with one or more storage media such as volatile and non-volatile computer memory such as RAM, PROM, EPROM, and EEPROM. The storage media may be encoded with one or more programs that, when executed on one or more processors and/or controllers, perform the required functions. Various storage media may be fixed within a processor or controller or may be transportable, such that the one or more programs stored thereon can be loaded into a processor or controller.

(130) The invention makes use of a wide light source illumination beam and enables effective aerodynamic diameter, effective optical diameter and shape factor all to be achieved in one shot. With this design, the sensor can be made with a small footprint and reasonable sensitivity (compared to a professional mass spectrometry system).

(131) The examples above show a measurement zone with continuous illumination. An alternative is to splitting a beam into two to define an envelope with two portions. The length of the measurement zone can then be increased in return for a higher sensitivity for aerodynamic diameter measurement, while retaining the shape determination function. For example a 4 cm beam may be split into a 2 cm laser+2 cm void+2 cm laser envelope with a measurement zone length of 6 cm.

(132) Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word comprising does not exclude other elements or steps, and the indefinite article a or an does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.